Method of forming routing data

ABSTRACT

Switching nodes of a communications network are assigned subnetworks and are interconnected to one another in any desired fashion. Stored in a source switching node is at least topology information on the node&#39;s own subnetwork and on the interconnection of the subnetworks. In addition, the source switching node is provided with the communications conditions which are required for the communications link to be set up. By reference to the topology information, a subset of switching nodes and connecting lines which satisfies the communications conditions is selected and a route to the destination switching node is determined. Included in this process is a route which, in the direction from the source switching node to the destination switching node, leaves at least one subnetwork once and returns to the subnetwork again in the further course of the route. The routing information is then formed from the route which has been determined.

BACKGROUND OF THE INVENTION

When links are routed in communications networks such as narrowband orATM networks (ATM=Asynchronous Transfer Mode), for example, there arebasically two alternative approaches, namely the “hop-by-hop routingmethod”, in which each transit node itself decides how to the forwardthe connection set-up request, and source routing in which the sourcenode S (to which the subscriber initiating the connection request isconnected) adds a route description to the connection set-up message,which description has to be followed by the transit nodes in order toarrive at the destination node D (to which the requested terminatingsubscriber is connected). This route description information is alsoreferred to as routing information or source routing information or,specifically in the case of ATM-PNNI networks, is referred to as DTLstack (=stack of Designated Transit List information elements).

Said ATM communications networks can be organized into numeroussubnetworks (“peer groups”), comprising physical switching nodes andphysical connecting lines (“physical links”). According to the PNNIprotocol, the nodes of a (hierarchically lowest) peer group determinefrom among them a so-called representative node (“peer group leader”)which represents the entire peer group in the form of a single, logical,model-like node (referred to as “logical group node” or else “parentnode”) in a peer group which is of a hierarchically higher level.

A hierarchically higher peer group is formed, comprising a plurality ofsuch parent nodes and the connecting lines which interconnect these in amodel-like fashion, in which case a model-like connecting line (alsoreferred to as “higher-level logical link”) between two such parentnodes thus represents all those physical connecting lines which connectin each case two physical boundary nodes from the hierarchy region ofthe two adjacent parent nodes and, in doing so, have assigned to them,thanks to administrative specifications and an agreement algorithm, thesame code in each case, referred to as aggregation token.

The hierarchy can continue recursively in further hierarchy levels: apeer group leader selection can also take place again in thehierarchically higher peer group. The peer group leader which isselected here represents again the entire hierarchy region establishedunder it in a peer group which is hierarchically at the next highestlevel, as if this hierarchy region were a single node. In this peergroup there are also logical, model-like connecting lines which areformed repeatedly, as described above.

A hierarchical model-like network in accordance with the PNNI protocol(for illustration purposes: 3-dimensional grid) is completed by addingfurther, purely logical connecting lines, the so-called “uplinks” whicheach connect, in accordance with the PNNI protocol, two nodes to oneanother (physically—if the node at the lower end of the uplink is aphysical node—or logically) from peer groups which are at hierarchicallydifferent levels.

Thus, an uplink (also referred to as “initial uplink”) leads from theboundary node of a hierarchically lowest peer group, which node isconnected to a boundary node in an adjacent peer group, to arepresentative node, the so-called “upnode”, i.e. to that representativenode “ancestor node” (i.e. parent node, grandparent node or great . . .grandparent node) of the adjacent boundary node which is a directlyneighboring node of precisely one specific ancestor node of the boundarynode on this side in a common peer group of a hierarchically higherlevel. Such an (initial) uplink results in all the ancestor nodes (ofthe boundary node on this side) which however each belong to ahierarchically lower peer group than the aforesaid common hierarchicallyhigher peer group, also each contribute an uplink (also referred to as“induced uplink”) to the aforesaid upnode [lacuna] the hierarchypattern.

The hierarchical structure, which is ultimately based on correspondingconfiguration data of the individual nodes, can be handled very flexiblyhere. In particular, the individual nodes of a great . . . grandparentpeer group can have different numbers of subhierarchy levels togetherwith the relevant peer groups.

The exchange, in accordance with PNNI protocol, of data packets, “hellopackets” and PNNI topology status data packets (“PNNI topology statepackets”—PTSPs) via so-called routing control channels ensures that eachphysical switching node of a hierarchically lowest peer group acquiresthe same knowledge of the topology of this group and of all the peergroups, including all the uplinks, which are located at a hierarchicallyhigher level than it in the hierarchy, and also the same knowledge ofthe usage factor of all the nodes and connecting lines contained in itas well as the same knowledge of its properties (accessibility,capabilities, features, costs).

The knowledge of the topology which is acquired can be stored in a nodein the form of a graph G1. In it, the respective current switching node(which has produced this graph G1 for itself) is not marked inparticular as the source node S.

If a terminal which is connected to the source node then requests to beconnected to the terminal of a specific destination address, the data inthe graph G1 which are exchanged per PNNI routing protocol make itpossible to determine that destination node D which indicates theaccessibility of the destination terminal and at the same time belongsto the hierarchically lowest possible peer group. On the basis of thegraph G1 it is possible to determine, in terms of a suitableminimization criterion, the best route from the starting node S to thedestination node D.

The ATM Forum Technical Committee Private Network Node Interface (PNNI)in the specification, version 1.0, Annex H does not, however, providethe possibility of also including in the route search advantageousbypasses via one or more peer groups with a return to the peer groupwhich has already been passed through, and as a result it is in themeantime not possible to fulfill a switching request appropriately.

These problems also occur in other communications networks, for examplenarrowband networks with source routing for implementing a PSTN (PublicSwitched Telephone Network). The topology information is evaluated onlyto the extent that routes are determined with the avoidance of bypasses.

SUMMARY OF THE INVENTION

The object of the method according to the invention consists indetermining a route, while taking into account the topology informationand the communications conditions relating to the nodes and connectinglines, and converting the route into routing information in such a waythat the largest possible variety of routes can be taken into account.

The switching nodes are assigned to subnetworks and interconnected toone another as desired. The subnetworks here can be individual localcommunications networks of different service providers or groups ofswitching nodes of a superordinate communications network. In a sourceswitching node there is topology information available on the node's ownsubnetwork and on the inter-connection with the subnetworks which arestored in the node or in a routing server. In addition, thecommunications conditions which are required for the communicationsconnection to be set up are available to the source switching node.

By referring to the topology information, a subset of switching nodesand connecting lines which satisfies the communications conditions isselected and a route to the destination switching node is determined.Included in this process is a route which, in the direction from thesource switching node to the destination switching node, leaves at leastone subnetwork once and returns to said subnetwork in the further courseof the route. The routing information is then formed from the routewhich is determined. The formation of the routing information is carriedout either in the switching node itself or in external devices, forexample routing servers, which can be connected to the switching node.The method according to the invention can be implemented, for example,in the Xpress switching nodes from Siemens AG.

On the basis of a topology graph which is based, for example, on thetopology information acquired by the PNNI routing protocol, a possiblyreduced topology graph is derived such that the remaining nodes andedges fulfill, inter alia, the conditions of the current connectionrequest. It is ensured that the topology graph is not reduced too muchso that bypasses are made possible in which it would be possible to passthrough nodes and edges which belong to a higher hierarchical level thanthe destination node D.

A routing algorithm which is carried out on this basis results in aroute which makes bypasses via other subnetworks, and it is thus interms of the minimization criterion applied the instantaneously bestroute—for instance because no routes without bypasses are possible owingto the instantaneous network usage factor, or if they are possible theyare less favorable. The latter is probable in particular if thenetworks/subnetworks (peer groups) are formed on the basis oforganizational view-points (owners, departments, . . . ), butgeographically cover the same area.

A communications connection which is set up according to this routinginformation will certainly also comprise bypasses which would beavoidable if a direct route were possible within a subnetwork whilecomplying with the communications conditions. However, as a result ofthe method according to the invention, blocking within the subnetwork isavoided. The number of permitted routes is substantially expanded andthe service providers of the communications network are presented withexpanded configuration possibilities by virtue of the use of additionalsubnetworks for a connection set-up.

The advantageous configuration of the invention includes the voluntarylimitation of the potential degree of bypasses. Here, the highest peergroups are advantageously removed, as it were voluntarily, from thetopology graph. This aspect is important if private and public networksform a common hierarchy and, for example for reasons of cost, the bypassvia public networks is to be prevented.

The switching nodes or subnetworks are advantageously implemented as anarrowband network or ATM network. The method according to the inventioncan, however, also be applied to hybrid forms of communicationsnetworks, with the result that only parts of the communications networkare implemented in this way.

A particularly simple way of implementing the method according to theinvention is obtained if at least some of the switching nodes operateaccording to the principles of the Private Network Node Protocol (PNNI).The switching nodes of a subnetwork are represented here by a complexswitching node in the topology information. The hierarchical structuringof parts of the communications network is made easier in this way. Inaddition, a reduction in expenditure is obtained in terms of thedetermination of routes, which is advantageously carried out accordingto the Dijkstra algorithm.

According to a further advantageous refinement of the method accordingto the invention, the connecting lines between two switching nodes arehandled separately in both traffic directions. This is effected in that,for example, each undirected edge is replaced by in each case twooppositely directed edges, each individually directed edge beingassigned both forward and rearward attributes.

In this way, the one directed edge can remain in the topology graphwhile the oppositely directed edge is removed if the communicationsrequirements are such that the connection set-up is made possible onlyin the one direction. Here, an oppositely directed downlink isincorporated into the topology graph for each uplink. On the other hand,the Dijkstra routing algorithm ensures that only monotonously directededge sequences result as routes.

Thus, a connection line can be passed through in one direction during aconnection set-up even if specific communications conditions are notfulfilled in the other direction.

When the communications conditions are not fulfilled by a switching nodeor a connecting line, making it impossible, for example, to set up aconnection via the previously determined route or to clear a connection,the respective switching node or the respective connecting line issignalled to the source switching node or an initial switching node ofthe corresponding subnetwork, in response to which the latter canperform a new route determining process.

During the route determining process, according to developments of theinvention, the routes with the shortest connecting lines or the lowestnumber of switching nodes to be passed through are selected. In thisprocess, if appropriate, geographic information on the switching nodesmay be accessed. If the topology information advantageously containscost-specific information, the route with the lowest costs is selected.During the selection of the most favorable route, the followingminimization criteria may therefore be taken into account individuallyor in a combined fashion:

Number of nodes to be passed through,

The sum of the distances between the nodes to be passed through,

The delay time of the transmission (cell transfer delay),

Variation in the delay time of a transmission (cell delay variation),

Transmission costs.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

1st Exemplary Embodiment

FIG. 1 shows an ATM communications network from the point of view of thesource switching node A.1,

FIG. 2 shows an ATM communications network from the point of view of thetransit switching node B.2 and

FIG. 3 shows an ATM communications network from the point of view of thefurther transit switching node C.1.

2nd Exemplary Embodiment

FIG. 4 shows a network topology of an ATM communications network,

FIG. 10 shows network topology represented by complex nodes,

FIG. 11 shows a topology graph for a node of subnetwork A,

FIG. 7 shows a topology graph for a node of subnetwork A withindependent traffic directions,

FIG. 8 shows a route which has been determined.

3rd Exemplary Embodiment

FIG. 9 shows a network topology of a narrowband communications network,

FIG. 5 shows network topology represented by complex nodes,

FIG. 6 shows a topology graph for a node of subnetwork A,

FIG. 12 shows a topology graph for a node of subnetwork A withindependent traffic directions,

FIG. 13 shows a route which has been determined.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 1st Exemplary Embodiment (PNNINetwork with Only Simple Nodes)

In a first exemplary embodiment (FIGS. 1 to 3), a graph G2 is determinedwhich, with reference to the scenario of an ATM communications networkdescribed at the beginning, is derived from the aforesaid graph G1 byremoving from graph G1 all those nodes and connecting lines which do notcomply with the communications conditions.

Subsequently, an optimum connection path is determined such thatbypasses via hierarchically higher peer groups with subsequent return tohierarchically lower peer groups which have already been passed throughare also taken into account. Here, the exit nodes and re-entry nodes forone and the same peer group are different; otherwise such a detour wouldconstitute an extremely unnecessary loop and not an optimum connectionpath.

Even if the following exemplary embodiment concentrates on detour-likeroutes, it must not be forgotten that, for normal cases in which a best,detourless route is available, the method according to the inventionalso finds said route and equally correctly forms the correspondingrouting information for it.

The resulting optimum route can in principle contain any desired numberof transitions from a hierarchically higher peer group to ahierarchically lower peer group and, vice versa, from a hierarchicallylower peer group to a hierarchically higher peer group, in which case ateach individual transition in principle any desired number, i.e. zero,one, two, . . . or n<=102 hierarchy levels may be skipped.

In accordance with the PNNI protocol, the connection set-up message isalso given the routing information as a consequence of informationelements, so-called “Designated Transit List information elements(DTLs)”, a preceding information element (repeat indicator) indicatingthe stack-like handling of these DTLs (push and pop operations). Here,each information element DTL contains the description of precisely oneroute through precisely one hierarchical peer group in the form of oneor more node links (also referred to as edge later) pair specificationsand a pointer which points to one of these node link pairs.

The route which is described from the uppermost information element DTLof the push-down storage starts here with the source node S and containsonly specifications relating to nodes and connecting lines in thehierarchically lowest peer group and ends, if appropriate, with thespecification of an uplink which leads to an upnode at which the routecontinues, specifically in the way described in the next lowest stackedDTL.

Each next lowest DTL in the push-down storage contains specificationsfor, in each case, one route through the hierarchically next highestpeer group, said specification starting with the specification of therelevant ancestor node of the source node, possibly followed by furthernode and connecting line specifications from the same peer group and apossible uplink specification as termination. The lowest DTL in thestack contains specifications relating to a route through thehierarchically highest peer group required, starting with thespecification of the relevant ancestor node of the source node andending at a node in whose hierarchy region the destination node with theconnected destination terminal is located.

The described design of the DTL push-down storage in accordance with thePNNI protocol initially gives the impression that it would be impossibleto take into account routes which comprise any desired sequences ofhierarchically higher and hierarchically lower nodes, and that it isactually indicated to design the algorithm for searching an optimumconnecting path in such a way that routes which such sequences (that isto say with bypasses via hierarchically higher peer groups) are excludedfrom the outset, as is the case in the specifications of the PNNIprotocol, version 1.0, Annex H.

However, the method according to the invention solves the problem ofdescribing a route containing bypasses in such a form that the rules ofthe PNNI protocol are satisfied.

It is characteristic of the solution of the first exemplary embodimentthat from a prescribed sequence of hierarchically higher andhierarchically lower nodes, in which sequence uplinks would certainlyalso have to be passed through in the downward direction, an equivalentsequence of nodes and connecting lines is derived which never runs in adescending fashion in terms of the hierarchy levels of these nodes andin which therefore uplinks never have to be passed through in thedownward direction. As an attribute of what is achieved in this way, oneand the same hierarchically higher (logical) node can occur repeatedlyin the sequence (loops) but, owing to the connecting lines which arespecified, it is clearly ensured that the exit and re-entry boundarynodes in the relevant child peer groups are always different, whichultimately means that one and the same physical node is never passedthrough more often than once.

Below, the determination of the best route in a switching node whichdetermines the route and the routing information is explained for thefirst exemplary embodiment:

A graph G3 is derived from the abovementioned graph G2 by removing allthe ancestor nodes of the source node S, likewise all the (horizontal)connecting lines which lead away from said nodes and which would leadfrom precisely these ancestor nodes to their adjacent nodes in thecorresponding hierarchically higher peer groups, and all the induceduplinks leading away from these ancestor nodes in the upward direction.

In accordance with the PNNI protocol, a best route is determined in aknown manner, for example using the Dijkstra routing algorithm, from thesource node S to the destination node D based on the graph G3, theuplinks remaining in the graph G3 having to be treated no differentlythan all the other (horizontal) connecting lines.

The sequence F1 in general notation:

node-n(=D), link-n−1, . . . , node-i+1, link-i, . . . , link-1,node-1(=S)

is obtained as best route.

It is in the nature of the Dijkstra routing algorithm that therespective best route is determined not only to a single, specificdestination node D, but rather to all nodes of the network, andafterwards the route of interest, for example to the destination node D,is picked up. By means of the Dijkstra algorithm, this route isinitially determined here in the form of the sequence F1.

Then, the sequence is turned about and the sequence F2:

node-1(=S), link-1, . . . , link-i, node-i+1, . . . , link-n−1,node-n(=D)

is formed.

The physical source node-1=S is naturally of the hierarchically lowestlevel. All the other nodes must, according to the invention, be inorder, as often as desired, hierarchically higher or hierarchicallylower physical or logical nodes. In particular, the destination nodenode-n=D must not necessarily be the hierarchically highest of the nodesoccurring in the sequence.

A link, link-i, proves to be horizontal if node-i and node-i+1 areassigned to the same hierarchy level, that is to say belong to the samehierarchical peer group. A link, link-i, proves to be an uplink in theupward direction (or downward direction) if the hierarchy level fromnode node-i is smaller (or larger) than the hierarchy level from nodenode-i+1.

According to the invention, from the sequence F2 a sequence F3 isderived, in which the nodes in the prescribed sequence never descend interms of their hierarchy level. Switching nodes and links from F2 are,if appropriate, replaced or canceled out by others here. For thispurpose, an auxiliary variable, referred to here as a CurrentNodeLevel,which is initialized with the hierarchy level of the node node-1=S, isused, together with a second Boolean auxiliary variable, referred tohere as BelowHighestReachedLevel, which is initialized with FALSE. In aniteration loop, all the components of the sequence F2 (the links and thenodes) are run through, starting at source node node-1=S, and in themeantime replacements or cancellations are carried out—see the followingalgorithm:

BelowHighestReachedLevel := FALSE;

current_node := node-1;//i.e. = Source Node S

 CurrentNodeLevel := Hierarchy level of the current_node;

for i:=1 step 1 to n−1 do

if hierarchy level of the node-i+1 is lower than

CurrentNodeLevel then

if BelowHighestReachedLevel = FALSE then

determine that ancestor node of node-i+1 whose hierarchy level is equalto the CurrentNodeLevel. Replace link-i, which is an uplink passedthrough in the downward direction, with the associated horizontal link(with the same aggregation token). How to do this: see Subtask-1 afterthis algorithm. Replace node i+1 with the ancestor node which has beendetermined. BelowHighestReachedLevel:= TRUE;

else

cancel link-i and node-i+1 from the sequence.

end

else

if BelowHighestReachedLevel = TRUE then

replace link-i with that assigned (induced) uplink or else horizontallink which starts from the level given by CurrentNodeLevel and leads tothe node i+1. How to do this: See Subtask-2 after this algorithm.Node-i+1 is retained unchanged in the sequence. BelowHighestReachedLevel:= FALSE;

else

retain link-i and note-i+1 unchanged in the sequence.

end

CurrentNodeLevel := Level of node-i+1;

end

Next i;

Subtask-1:

Determine the associated horizontal link in the hierarchically higherpeer group for a prescribed uplink:

The graph G1 has m links (horizontal links and uplinks taken together).The number k from the set 1,2, . . . , m represents a pointer to theinteresting information relating precisely to one link (for example itsidentity specifications). In particular, assume that there is a tableRelationTbl with m elements. The elements represent the assignment chainfrom the initial uplink to the possibly induced uplink, to the uplinkwhich is possibly derived therefrom again, etc., to the horizontal linkinduced therefrom in a hierarchically higher peer group:

RelationTbl[j₁] := j₂; // if there is no value k of 1 to m withRelationTbl (k):=j₁, j₁ is the initial uplink

RelationTbl[j_(q−1)] := j_(q);

. . .

RelationTbl[j_(r−1)] := j_(r);

. . .

RelationTbl[j_(s−1)] := j_(s);

RelationTbl[j_(s)] := 0;

which means:

link- j₁ is uplink and induces link- j₂

link- j_(r−1) is uplink and induces link- j_(r)

link- j_(s−1) is uplink and induces link- j_(s)

link- j_(s) is a horizontal link.

// If there is no value j_(s−1) of 1 to m with RelationTbl(j_(s−1)):=j_(s), but there is an entry RelationTbl[j_(s)] := 0, j_(s)is a horizontal link in a hierarchically lowest peer group.

Assuming that the link-i to be replaced corresponds to j_(q−1), thetable RelationTbl is run through until RelationTbl[j_(s)] := 0 isarrived at. j_(s) defines the horizontal link to be used.

Subtask-2:

For all the m links of the graph G1 there is a table of the type:

LinkLevelTbl[k] = lowest hierarchy level of the two boundary nodes ofthe link k; for all k=1, . . . m. link-i is designated by j_(q−1). Thetable RelationTbl is run through starting from RelationTbl[j_(q−1)] inorder to pass from one link to the next link, and, in the process,CurrentNodeLevel is continuously compared with the entries inLinkLevelTbl. Assuming that the value of CurrentNodeLevel is equal tothe value of LinkLevelTbl[j_(r−1)], j_(r−1) identifies the searched-forlink which is to replace link-i.

From the sequence F3, it will be assumed that a sequence F4 is formed,for example as follows:

for i:=1 step 1 to n-1 do

if link-i=uplink then

insert after link-i that ancestor node from source node S which is ofthe same hierarchy level as node-i+1. In turn, insert after thathorizontal link H which is assigned to the link-i by virtue of theidentical aggregation token (the RelationTbl is run through starting atj₁=link-i and H=j_(s) is found).

end

next i;

From F4, a sequence of DTLs is formed by breaking up the sequence F3after each uplink, and from each subsequence which is generated in thisway a DTL information element with PNNI protocol-compatible syntax isformed, which completely describes the task according to the inventionfor the source node S.

According to the invention, a loop can be integrated (in the terms ofthe method according to the invention this is a bypass via one or morepeer groups with return to a peer group which has already been passedthrough at a re-entry node which has not yet been passed through) evenwhen the information elements of the routing information are fulfilledagain when the connection set-up message arrives in a physical switchingnode (transit node) to be passed through. This is clarified below withreference to the arrival of a connection set-up message in the firstphysical switching node of a hierarchically lowest peer group:

If the peer group which is the hierarchically lowest at a given momentis exited when a connection set-up message is being passed on, therelevant uppermost DTL in the stack must have previously been removed.If even a certain hierarchy range is exited, all those uppermostinformation elements DTL in the stack which contain routing sectionsthrough the respective peer groups of the hierarchy region to be exitedmust previously have been removed. If a peer group which ishierarchically the lowest is re-entered when a connection set-up messageis being passed on, new routing sections must be determined and newrelevant DTLs must be formed. The pointers in the individual DTLs mustalways have been set and/or moved forward in such a way that when aconnection set-up message is received the pointers of all the receivedDTLs each point to a node link pair which contains either the receivedphysical node which is the lowest in the hierarchy or else one of itsancestor nodes.

The boundary node (S′) determines, as entry node into a further peergroup, a new best route section as far as a destination node D′. Thedestination node D′ is to be taken from the node link pair which followsthe node link pair in the DTL which is uppermost in the stack of thereceived DTLs and to which the relevant pointer points.

If this is not possible because the pointer is already pointing to thelast node link pair, the same applies with respect to the next lowestDTL in the stack, and so on. It is within the spirit of the inventionthat the PNNI protocol prescribes that the received link specification,namely how a node D′ is arrived at, must also be complied withcompletely. Any attempt, for example to get there in a better way, couldcertainly produce a better route section, but at the same time couldalso lead to the hierarchy region represented by the destination node D′not being entered at the expected boundary node, from where thecontinuation of the connection set-up could finish up in a dead end.That is to say in addition to D′ the horizontal link Link-to-D′ is alsodetermined. Link-to-D′ is taken from the node link pair which belongs tothat DTL in which D′ is located and to which the relevant pointer pointsduring the reception of the DTL.

The boundary node S′ which considers that it is only a transit node forthe present connection set-up request and that the received DTLpush-down storage is incomplete, therefore forms, on the basis of itsown graph G1, a possibly reduced graph G1′, as follows:

All the nodes—together with the adjacent links—with a hierarchy levelwhich is greater than or equal to the hierarchy level of the node D′ areremoved from the graph G1, but not D′ itself and also not those uplinksfor which D′ is an upnode and are at the same time assigned to theLink-to-D′. That is to say for all uplinks which have D′ as an upnodethe following test is made:

As in Subtask-1, a J_(q−1) can be assigned to an uplink. The tableRelationTbl will be run through starting from RelationTbl[j_(q−1)] untilan entry RelationTbl[j_(s)] :=0 is arrived at. If j_(s) corresponds toLink-to-D′, the uplink can remain in the graph G1′. Otherwise, it isremoved. The graphs G2′ and G3′ are formed on the basis of G1′, bycomplete analogy with the way that the source node S formed the graphsG2′ and G3′, and a DTL stack is determined as described above, S′performing the function of S and D′ performing the function of D (seedescription above).

All the DTLs, with the exception of the last-but-one (which contains D′)are transferred from the resulting DTL push-down storage and the DTLpush-down storage to be passed on is thus completed.

The formation of the routing information in the switching node of anillustrated ATM communications network will be explained with referenceto FIGS. 1 to 3. Here, FIGS. 1 to 3 show one and the same ATMcommunications network for a routing search and the formation of routinginformation, but viewed from different switching nodes.

The hierarchy structure of the ATM communications network shows threesubnetworks TA, TB, TC by way of example. The first subnetwork TAcomprises the physical nodes A.1 . . . 6. For the connection set-up inquestion, the node A.1 is the source switching node and the node A.6 isthe destination switching node. However, these source and destinationnodes do not have to be located in the same peer group (subnetwork); inaddition, bypasses can be made only through a transit node. A furthersubnetwork TB comprises the nodes B.1 . . . 5 and an additional furthersubnetwork TC comprises the nodes C.1 . . . 4. The subnetworks TA, TB(peer groups of the lowest hierarchy level) are combined at a higherhierarchy level to form a network group TAB (peer group of a higherhierarchy level) and are each represented by a logical node A, B. At astill higher hierarchy level, this network group TAB (peer group) iscombined with the further additional subnetwork TC to form a networkgroup TABC, one logical node AB representing the network group of ahigher hierarchy level TAB and one logical node C representing thefurther additional subnetwork TC.

The nodes are connected to one another by means of physical connectinglines (physical links). Links pb1,2,3 and pc1,2 between nodes ofdifferent subnetworks are assigned additional information.

Legend:

Initial vector

p=physical link,

h=horizontal link,

u=initial uplink,

U=induced uplink

pb1, hb1, and ub1, or pb2, hb2, and ub2 or pb3, hb3, and ub3 or pc1,uc1, Uc1 and hc1 or pc2, uc2, Uc2 and hc2 are distinguished, by way ofexample, with a respective identical aggregation token.

The nodes in which the routing information is formed according to FIGS.1 to 3 see only in each case the peer groups outlined by thick lines(the knowledge base stored in the respective node comprises informationon these peer groups). Instead of the physical connecting lines whichlead out of the peer group which is lowest in the hierarchy, they seethe relevant assigned uplinks. Only the respective boundary nodesthemselves know about this assignment, but they do not communicate thisinformation to the other nodes of the peer group.

A route from the source node A.1 to the destination node A.6 is searchedfor. The connecting lines pa5, pa6 which would permit a direct routefrom the source node A.1 to the destination node A.6 are blocked. Thephysical path which the connection set-up should take is shown by athick line.

Activity of the source node A.1:

In the source node A.1, the graph G1 will be stored in the form of alist of links together with their boundary nodes, i.e.

G1(A.1)—see FIG. 1:

(pa2: A.2, A.1), (pa3: A.3, A.1), (pa4: A.4, A.5), (pa5: A.5, A.1),(pa6: A.6, A.5),

(ub1: A.2, B), (ub2: A.3, B), (ub3: A.4, B),

(uc1: A.5, C), (uc2: A.6, C).

(hb1: B, A), (hb2: B, A), (hb3: B, A),

(Uc1: A, C), (Uc2: A, C),

(hc1: C, AB), (hc2: C, AB),

The blocked lines are removed, specifically (pa5: A.5, A.1), (pa6: A.6,A.5), and

the graph G2 (A.1) is determined:

(pa2: A.2, A.1), (pa3: A.3, A.1), (pa4: A.4, A.5),

(ub1: A.2, B), (ub2: A.3, B), (ub3: A.4, B),

(uc1: A.5, C), (uc2: A.6, C).

(hb1: B, A), (hb2: B, A), (hb3: B, A),

(Uc1: A, C), (Uc2: A, C),

(hc1: C, AB), (hc2: C, AB),

All the ancestor nodes together with the adjacent lines are removed,namely

(hb1: B, A), (hb2: B, A), (hb3: B, A),

(Uc1: A, C), (Uc2: A, C),

(hc1: C, AB), (hc2: C, AB),

and thus graph G3 (A.1) is determined:

(pa2: A.2, A.1), (pa3: A.3, A.1), (pa4: A.4, A.5),

(ub1: A.2, B), (ub2: A.3, B), (ub3: A.4, B),

(uc1: A.5, C), (uc2: A.6, C).

The application of the Dijkstra routing algorithm produces the sequenceF1:

Destination Node D = A.6, uc2,C, uc1,A.5, pa4, A.4, ub3, B, ub1, A.2,A.1=Source Node S.

The reverse order = F2 is:

Source Node S=A.1, pa2,A.2, ub1, B, ub3, A.4, pa4, A.5,

uc1, C,uc2, A.6 = Destination Node D.

The sequence F3 is determined:

A.1, pa2, A.2, ub1, B, hb3,A, Uc1, C, hc2, AB.

The sequence F4 is determined:

A.1, pa2, A.2, ub1, A, hb1, B,hb3,A, Uc1,AB, hc1, C, hc2, AB.

The information elements of the DTL push-down storage are derived fromthis. The sequence F4 is split up after each uplink and from each of thesubsequences produced an information element DTL is formed whichindicates how they are transmitted to the next physical node. Thepointer points to the x-th bracketed node link pair.

1. DTL: (A.1, pa2), (A.2, ub1), pointer=2

2. DTL: (A, hb1), (B, hb3), (A, Uc1), pointer=1

3. DTL: (AB, hc1), (C, hc2), (AB,x′00 00 00 00), pointer=1

The activity of the transit node B.2 (entry node in the furthersubnetwork TB):

The transit node B.2 receives the following routing information ri:

1. DTL: (A, hb1), (B, hb3), (A, Uc1), pointer=2

2. DTL: (AB, hc1), (C, hc2), (AB,x′00 00 00 00), pointer=1

Node B.2 has stored the network which it can see, as a graph G1(B.2)—see FIG. 2, outlined by thick lines:

(pb4: B.1,B.2), (pb5: B.1,B.5), (pb6: B.4,B.5), (pb7: B.3,B.4), (pb8:B.2,B.3),

(ua1: B.2, A), (ua2: B.3, A), (ua3: B.4, A),

(hb1: B, A), (hb2: B, A), (hb3: B, A),

(Uc1: A, C), (Uc2: A, C),

(hc1: C, AB), (hc2: C, AB).

Because the traffic is transit traffic, G1′ (B.2) is formed bydetermining first D′ (B.2) and Link-to-D′ (B.2):

D′ (B.2) =A; Link-to-D′ (B.2) =hb3.

In the transit node, all the nodes of the hierarchy level which isgreater than or equal to the logical node A and the relevant adjacentlinks are removed from the graph G1 (B.2), with the exception however ofD′ (B.2) =A itself and excepting those uplinks for which A is an upnodeand which are correlated with Link-to-D′ (B.2)=hb3.

This produces the graph G1′ (B.2):

(pb4: B.1,B.2), (pb5: B.1,B.5), (pb6: B.4,B.5), (pb7: B.3,B.4), (pb8:B.2,B.3),

(ua3: B.4, A)

Since no blockages at the transit node B.1 are known, it is true thatgraph G1′ (B.2) =graph G2′ (B.2). Since it is not possible for anyancestor nodes of B.1 to be further away from this, it is true that G1′(B.2) =G2′ (B.2) =G3′ (B.2).

The Dijkstra algorithm which is applied yields a sequence F1:

D′ (B.2)=A, ua3, B.4, pb7, B.3, pb8, B.2=S′ (B.2)

In the reverse order this results in F2:

S′ (B.2)=B.2, pb8, B.3, pb7, B.4, ua3, A=D′ (B.2)

The operation to form F3 does not produce any changes, i.e. sequence F2=sequence F3.

Sequence F4 is formed from sequence F3:

S′ (B.2)=B.2, pb8, B.3, pb7, B.4, ua3, B, hb3, A=D′ (B.2)

The information elements DTLs of the routing information ri are formedfrom sequence F4:

1. DTL: (B.2, pb8), (B.3, pb7), (B.4, ua3), pointer=2

2. DTL: (B, hb3), A=D′ (B.2), pointer=1

the last (=2.) DTL of which is not transferred.

The following routing information ri is thus sent in the DTL push-downstorage format from the entry node B.2 to the further node B.3 in thefurther subnetwork TB:

1. DTL, newly formed:

(B.2, pb8), (B.3, pb7), (B.4, ua3), pointer=2

2. DTL, received and further processed:

(A, hb1), (B, hb3), (A, Uc1), pointer=2

3. DTL, received and further processed:

(AB, hc1), (C, hc2), (AB,x′00 00 00 00), pointer=1

Activity of the transit node A.4 in the first subnetwork TA:

The node A.4 receives the following routing information ri:

1. DTL: (A, hb1), (B, hb3), (A, Uc1), pointer=3

2. DTL: (AB, hc1), (C, hc2), (AB,x′00 00 00 00), pointer=1

Node A.4 has stored the network which it can see as a graph G1 (A.4)which corresponds to the graph G1 (A.1) stored by the source node A.1,see above and see FIG. 1 (outlined in thick lines).

Because the traffic is transit traffic, G1′ (A.4) is formed by

firstly determining D′ (A.4) and Link-to-D′ (A.4):

D′(A.4) = C

Link-to-D′ (A.4) = hc1.

All the nodes from the hierarchy level which is greater than or equal tothe hierarchy level of D′ (A.4) = C, and the relevant adjoining links,are removed from the graph G1 (A.4), but with the exception of D′ (A.4)= C itself and excepting those uplinks for which D′ (A.4) = C is anupnode and which are correlated with Link-to-D′ (A.4) = hc1.

Graph G1′ (A.4) is produced:

(pa2: A.2, A.1), (pa3: A.3, A.1), (pa4: A.4, A.5), (pa5: A.5, A.1),(pa6: A.6, A.5),

(ub1: A.2, B), (ub2: A.3, B), (ub3: A.4, B), (uc1: A.5, C),

(hb1: B, A), (hb2: B, A), (hb3: B, A),

(Uc1: A, C)

The blocked links are removed and this results in graph G2′ (A.4):

(pa2: A.2, A.1), (pa3: A.3, A.1), (pa4: A.4, A.5),

(ub1: A.2, B), (ub2: A.3, B), (ub3: A.4, B),

(uc1: A.5, C),

(hb1: B, A), (hb2: B, A), (hb3: B, A),

(Uc1: A, C)

When all the ancestor nodes still contained are removed from this,together with the adjoining connecting lines, graph G3′ (A.4) isproduced:

(pa2: A.2, A.1), (pa3: A.3, A.1), (pa4: A.4, A.5),

(ub1: A.2, B), (ub2: A.3, B), (ub3: A.4, B),

(uc1: A.5, C).

By means of Dijkstra routing algorithm, the following sequence F1 isdetermined here as the best route from the transit node A.4 to therepresentative C of the additional further subnetwork TC:

D′ (A.4)=C, uc1, A.5, pa4, A.4=S′ (A.4)

By reversing the sequence, the sequence F2 is obtained:

S′ (A.4)=A.4, pa4, A.5, uc1, C=D′ (A.4)

Since the sequence F2 is never descending in terms of the hierarchylevel of the nodes which occur, the operations for forming the sequenceF3 do not produce any changes: F3=F2.

The sequence F4 is acquired from the sequence F3, namely:

S′ (A.4)=A.4, pa4, A.5, uc1,AB, hc1, C=D′ (A.4)

The following information elements DTLs of the routing information riare derived from the sequence F4:

1. DTL: (A.4, pa4), (A.5, uc1), pointer=2

2. DTL: (AB, hc1), (C, x′00 00 00 00), pointer=1

the last (=2.) DTL of which is not transferred.

The following DTL push-down storage contents are thus transferred fromthe transit node A.4 to the node A.5:

1. DTL, newly formed: (A.4, pa4), (A.5, uc1), pointer=2

2. DTL, received and further processed: (A, hb1), (B, hb3), (A, Uc1),pointer=3

3. DTL, received and further processed: (AB, hc1), (C, hc2), (AB,x′00 0000 00), pointer=1

Activity of the transit node C.1 in the additional further subnetworkTC:

The node C.1 receives the following routing information ri:

1. DTL: (AB, hc1), (C, hc2), (AB,x′00 00 00 00), pointer=2.

The node C.1 has stored the network which it can see, as a graph G1(C.1), see FIG. 3:

(pc3: C.1, C.2), (pc4: C.2, C.3), (pc5: C.3, C.4), (pc6: C.1, C.4)

(uab1: C.1, AB), (uab2: C.4, AB)

(hc1: C, AB), (Hc2: C, AB)

Because the traffic is transit traffic, G1′ (C.1) is formed by firstlydetermining D′ (C.1) and Link-to-D′ (C.1):

D′ (C.1) = AB

Link-to-D′ (C1) = hc2.

All the nodes from a hierarchy level which is greater than or equal tothe hierarchy level of D′ (C.1) = AB, and the relevant adjoining links,are removed from the graph G1 (C.1), but with the exception of D′ (C.1)= C itself and excepting those uplinks for which D′ (C.1) = AB is anupnode and which are correlated with Link-to-D′ (C.1) = hc2.

Graph G1′ (C.1) is produced:

(pc3: C.1, C.2), (pc4: C.2, C.3), (pc5: C.3, C.4), (pc6: C.1, C.4),

(uab2; C.4, AB)

Since the node C.1 does not find any blocked connecting lines (these arelocated in the first subnetwork TA), G1′ (C.1)=G2′ (C.1).

Because no ancestor nodes relating to node C.1 can additionally beremoved from this, it is true that:

G1′ (C.1)=G2′ (C.1)=G3′ (C.1)

Using the Dijkstra routing algorithm, the node C.1 will determine asbest route the sequence F1:

D′ (C.1)=AB, uab2, C.4, pc6, C.1=S′ (C.1)

By reversing the order, sequence F2 is obtained:

S′ (C.1), pc6, C.4, uab2, AB=D′ (C.1)

Since the sequence F2 is never descending in terms of the hierarchylevel of the nodes which occur, the operations to form the sequence F3do not produce any changes: F3=F2.

The sequence F4 is acquired from the sequence F3, namely:

S′ (C.1)=C.1, pc6, C.4, uab2,C,hc2, AB=D′ (C.1)

The following information elements DTL of the routing information ri arederived from the sequence F4:

1. DTL: (C.1, pc6), (C.4,uab2), pointer=2

2. DTL: (C, hc2), (AB,x′00 00 00 00) pointer=1 the last (=2.) DTL ofwhich is not transferred.

The following DTL stack is thus transferred from the node C.1 to thefurther node C.4 in the additional further subnetwork TC:

1. DTL, newly formed: (C.1, pc6), (C.4, uab2), pointer=2

2. DTL, received and further processed: (AB, hc1), (C, hc2), (AB,x′00 0000 00), pointer=2.

Activity of the re-entry node A.6 for the second re-entry into the firstsubnetwork TA:

The node A.6 receives the following routing information ri:

1. DTL: (AB, hc1), (C, hc2), (AB,x′00 00 00 00), pointer=3.

If node A.6 detects that the destination terminal is directly connectedto it, it transmits to it the connection set-up message along therelevant UNI interface (no longer PNNI interface), in which case, inaccordance with the UNI protocol, no information elements DTLs aretransmitted at the same time. The route is shut down.

2. Exemplary Embodiment (PNNI Network with Simple and Complex Nodes):

A second exemplary embodiment shows a communications network in whichthree different network providers A, B and C connect their networksreciprocally with physical lines (connecting lines p): (see FIG. 4)

in New York (NY): A to C, and B to C,

in Chicago (Ch): B to C,

in Atlanta (Atl): A to B,

in Los Angeles (LA): A to B, and B to C.

Blocked lines are crossed out. A subnetwork A terminal in Atlanta wishesto set up a connection to a destination terminal, also in network A, inLas Vegas.

The dotted line shows the physical course of a route, starting at theAtlanta node of the network A and ending in the Las Vegas node, also ofthe network A. The determination of the routing information, or of thisroute, is shown below.

According to the PNNI concept, the three subnetworks A, B, C can each beconceived as the logical group node of a hierarchically higher peergroup, the sizes of the three networks indicating that they can all berepresented as complex nodes: comprising an imaginary nucleus in thecenter of a large circle, ports at the edge of a large circle, portnucleus connecting lines (spokes between port and nucleus), andport-port connecting lines (bypass exceptions between every two ports).The physical cross-connecting lines between the three networkscorrespond to so-called horizontal links (h) (see FIG. 10).

Each node of the subnetwork A thus has the following (identical) networkstructure, in accordance with PNNI, stored as topology information (seeFIG. 11). There is a visual representation of its topology databasewhich is in accordance with PNNI. It comprises essentially the “precise”diagram of the subnetwork A as well as the simplified, aggregated nodeswhich are represented as complex nodes and which represent each of thesubnetworks A, B, C as entireties.

Creating a comprehensive topology graph: According to the invention, thefollowing topology graph is derived from the topology database which isin accordance with the PNNI, said topology graph containing onlydirected, i.e. unidirectional edges. Each node (the simple nodes of thesubnetwork A, the port nodes and nucleus nodes of the complex nodes)receives a node number i (1<=i<=m), and each (unidirectional) edge (alsoreferred to as link) receives an edge number j (1<=j<=n).

The topology graph (see FIG. 7) is stored as a table whose elements areindexed by edge number j and which contain the two boundary node numbersof the edge j, i.e. that node at which the directed edge begins and thatnode to which it leads. The edge direction corresponds to the “outgoingdirection” in the sense in which it is used in the horizontal linksPTSE.

However, further information is also given. For this reason, thefollowing entries are made per edge j:

LinkTable[j] .FromNode LinkTable[j] .ToNode LinkTable[j].ForwardAttributesPtr LinkTable[j] .BackwardAttributePtr LinkTable[j].SpokeOrBypass // TRUE/FALSE LinkTable[j] .Included // TRUE/FALSELinkTable[j] .InducedLink // Edge number of the induced // uplink edgeor induced // (outgoing) horizontal edge // or 0 LinkTable[j] .MyUpLink// Number of the reciprocal // edge, if j is downlink, // otherwise=0LinkTable[j] .AggregationToken // 4 bytes LinkTable[j] .Type // Valuesare: horizontal, bypass // uplink, downlink, spoke, LinkTable[j] .PortId// horizontal, uplink

The topology graph also comprises m nodes. For each node i, thefollowing information is installed:

NodeTable[i] .Type // Values are: port, nucleus, simple NodeTable[i].PortId // 4 byte port ID>0 if // port node NodeTable[i] .MyNucleusNode// Associated nucleus node // number if Type=Port, // otherwise 0)NodeTable[i] .AncestorNode // Ancestor node number or 0 // if noancestor node // present. Its type is // either simple or nucleus.NodeTable[i] .HierarchyLevel // Range: 0 to 104 NodeTable[i].TransitRestricted // TRUE/FALSE NodeTable[i] .NextPortNode // Chain ofport nodes: // The nucleus node // points to the first // port node, thefirst // one to the second, // the last to 0. // The value is also 0, //if i is a simple // node. NodeTable[i] .NodeId // The same value with //port and nucleus // nodes.

The following information can be derived from these entries byconversion:

NodeTable[i] .CountIncomingLinks // Number of // incoming edgesNodeTable[i] .IncomingLinkTable [j] // Incoming edge // j in node i

A node number i is created:

a) NodeTable[i] .Type = Simple

This assignment is made after the evaluation of a nodal information PTSEof a simple node. It is assigned the NODE ID of this nodal informationPTSE by NodeTable[i] .NodeId.

b) NodeTable[i] .Type = Nucleus

This assignment is made after the evaluation of a nodal information PTSEof a complex node. It is assigned the NODE ID of this nodal informationPTSE with NodeTable[i] .NodeId.

c) NodeTable[i] .Type = Port

This assignment is made after evaluation of a nodal information PTSE ofa complex node and of a relevant (outgoing) horizontal link PTSE oruplink PTSE which contains a port ID. Each port node stores its nucleusnode number per NodeTable [i] .MyNucleusNode, and its PORT ID perNodeTable[i] .MyPortID.

The node numbers of the port nodes relating to a complex node are,starting at their nucleus node, interlinked forward per NodeTable[i].NextPortNode entries.

The switching node (= simple node) which implements this has its ownnode number stored specially. It is referred to below by S (S asstarting node). The node numbers of its ancestor nodes (they are eitherof the type = simple or nucleus) are determined as follows andinterlinked beginning at i=S by means of NodeTable[i] .AncestorNode.Embedded in the nodal information PTSE is a HigherLevelBindinginformation group which contains the node ID of the next ancestor node.This interlinking is set up by searching and comparing with theNodeTable[i] .NodeID entries.

An edge, i.e. an edge number, is created, in which case: a) LinkTable[j].Type = Horizontal This assignment is made after evaluating a horizontallink PTSE, whose port is stored in LinkTable[j] .PortId. The oppositeedge here is based on the existence of a second complementary horizontallink PTSE. Two horizontal links PTSE are complementary to one another iftheir originating node and remote node specifications (starting node anddestination node) are reciprocal and both contain one and the sameaggregation token. Each “horizontal” edge is assigned forward andrearward attributes. The forward attributes are the outgoing resourceavailability information of the respective horizontal link PTSE. Therearward attributes are the outgoing resource availability informationof the complementary horizontal link PTSE.

The “horizontal” edge j starts at a node whose number = NodeTable[i].FromNode and which is determined as follows:

If the starting node of the horizontal link PTSE is a simple node, theinitial node number is determined by this simple node alone. If thestarting node is however a complex node, the initial node number isdetermined by the complex node and additionally by the port ID from thehorizontal link PTSE. The initial node is thus a port node.

The horizontal edge ends at a node whose number = LinkTable[j] .ToNode,which is determined as follows:

If the destination node of the horizontal link PTSE is a simple node,the end node number is determined by this simple node (and its NODE ID)alone. However, if the destination node is a complex node, the end nodenumber is determined only by the additional remote port ID. It is thus aport node. A horizontal edge j has, in particular, set LinkTable[j].InducedLink = 0.

b) LinkTable[j] .Type = Uplink

This assignment is made after evaluation of an uplink PTSE. The forwardattributes are the outgoing resource availability information from theuplink PTSE. The rearward attributes are located in an ULIA informationgroup from the uplink PTSE.

The uplink edge starts at a node whose number = LinkTable[j] .FromNode,which is determined as follows: If the starting node of the uplink PTSEis a simple node, the initial node number is determined by the simplenode alone. However, if the starting node is a complex node, the initialnode number is determined by the complex node plus the port ID from theuplink PTSE. It is then a port node.

The uplink edge ends at a node whose end node number is determined asfollows:

If the upnode of the uplink PTSE is a simple node, the end node numberis determined by this simple node alone. If the upnode is however acomplex node, the end node number is determined by the complex node plusa port ID, which is determined as follows:

Firstly, that ancestor node which has the same hierarchy level as saidcomplex upnode is determined. Then, the topology database is searched atthis ancestor node for that horizontal link PTSE whose remote nodespecification is the NODE ID of the complex upnode and whose aggregationtoken is identical with that of the uplink PTSE. Under Remote Port ID inthis horizontal link PTSE there is the searched-for port ID. The endpoint of the uplink edge is then a port node. An uplink edge j has set,in particular, LinkTable[j] .InducedLink =j*>0, pointing to an inducedfurther uplink or to an induced horizontal link.

The starting node and end node of the complementary downlink edge arereciprocal with respect to these specifications of the uplink edge. Adownlink edge j has, in particular, a reference to the complementaryuplink edge j*: LinkTable[j] . MyUplink = j*>0. All the other edges have0 entered here.

c) LinkTable[j] .Type = DownLink

A complementary downlink edge is additionally created for the uplinkedge, with reciprocal boundary nodes and reciprocal forward and rearwardattributes.

d) LinkTable[j] .Type = Spoke

This assignment is made after evaluation of a nodal information PTSE ofa complex node plus an associated nodal state parameter PTSE with inputport x=0 and output port y=0 (default spokes). With regard to a complexnode, two complementary edges are created between a port node and anucleus node.

The forward attributes are identical to the rearward attributes. This istrue for the port-nucleus connecting lines and for the nucleus-portconnecting lines. After all the spoke edges have been created in thisway, it will be possible to correct the forward and rearward attributesof individual (default) spokes on the basis of further nodal stateparameter PTSEs with x=0 and y>0 or x>0 and y=0. If, owing to a nodalstate parameter PTSE with input port x>0 and output port y=0, theforward attributes of a port-nucleus connecting line are modified, therearward attributes of the complementary nucleus-port connecting lineare also modified with the same values.

d) LinkTable[j] .Type = Bypass

This assignment is made after evaluation of a nodal information PTSE ofa complex node plus an associated nodal state parameter PTSE with inputport x>0 and output port y>0 (bypass exception). There is provision fora nodal state parameter PTSE also to be prescribed for the reversedirection from the input port y to the output port x. Each of these twoPTSEs contains the forward attributes which can serve simultaneously asrearward attributes of the complementary edge. A pair of bypassexceptions is provided between two ports whenever the two ports liegeographically as near to one another as possible.

All the edges receive, in addition to the fact that an edge number j iscreated for them, the marking LinkTable[j] .Included := TRUE If theseentries have been made, the topology information is available in thegraph G1.

Reduction of the topology graph G1:

The topology graph which has been created so far is the mostcomprehensive graph and is then newly created if PTSEs which aredecisive for its creation have disappeared from the topology database ornew ones have been added.

Before a specific determination of a route which includes very differentparticular features and communications conditions, certain edges j areexcluded from this, specifically per: LinkTable[j] .Included :=FALSE

a) Avoiding transit-restricted nodes:

According to a transit-restricted bit of its nodal information PTSE, anode may not be available for transit switching services. The result ofthis is that for relevant simple, port or nucleus nodes i, NodeTable[i].TransitRestricted:=TRUE is set. An edge j is excluded if at least oneof its two boundary nodes has set this value to TRUE and it is at thesame time neither a source switching node nor destination switching nodeof the route. The destination switching node D is always of the simpleor nucleus type, never port. If the destination node D is of the nucleustype and this, like all the associated port nodes, has setTransitRestricted:=TRUE, neither the nucleus node nor the associatedport nodes are excluded from the topology graph. In the example: node 22and all the adjoining edges

b) Non-metric peripheral conditions not fulfilled

Individual edges could be provided only for a subset of servicecategories (CBR, ABR, UBR, VBR-rt, VBR-nrt), which does not belong tothat of the connection request currently in question. The communicationsconditions provided are non-metrical quality-of-service parameters (max.error rate) whose individual edges cannot satisfy the requirements ofthe connection request. They are excluded from graph G1.

c) Generic Call Admission Control (G-CAC);

At the given time, individual edges might not have the necessarybandwidth available, and therefore not pass the Generic Call AdmissionControl test.

LinkTable[i] .ForwardAttributePtr and LinkTable[i] .BackwardAttributePtrcomprise the current usage factor information of the individual edgeswhich is required here. The edges which do not fulfill thecommunications conditions are excluded from graph G1. In the example,the unidirectional edge 56:21-19 is excluded on this occasion. Thetopology graph which is reduced owing to a), b) or c) is designated asgraph G2.

d) Ancestor node

All the ancestor node of the source node S of the route are excluded,i.e. all the edges which start or end at them. All these ancestor nodesare in the sequence {NodeTable[i] .AncestorNode} starting with i=s. Inthe example, nucleus and port nodes of subnetwork A, and all adjoiningedges, are excluded. The graph which is reduced in this way is referredto as G3.

e) Possible limitation on the degree of bypasses:

The node determined as destination switching node D of the route is onewhose PNNI hierarchy level is equal to h [0<h<=104). A networkadministrator could possibly prescribe an integer x such that onlybypasses which do not exit the hierarchy range of an ancestor peer groupof the level H=Max(0, h-x) are permitted. All those edges of which atleast one boundary node has an excessively high hierarchy range, i.e. avalue <H, are excluded. In the example, no edges are excluded on thisoccasion. The graph which is restricted in this way is designated G4.

Dijkstra best route calculation

Based on the reduced topology graph G3 or G4, a Dijkstra best routecalculation is carried out, starting from precisely one starting nodeS=MyOwnNodeNumber (S=21) to any destination node. This iteration stopsas soon as the best route has been found for the desired destinationnode D (D=17).

The minimization criterion, referred to below as distance, is, forexample, the minimum administrative weight sum. An iteration of theDijkstra algorithm includes two substeps:

a) Update of the distance to the source switching node at all the nodesat which this distance value is not yet definitive.

b) Determination of the next node for which the distance entered at agiven time is the definitive one, in which case it would also be definedwhich node is the direct preceding node for this node and which edge isthe preceding edge in the direction of the source switching node.

It is also to be noted that only those edges which end in the respectivenode are considered.

Metric peripheral conditions: Both a Cell Transfer Delay and a CellDelay Variation are valid as metric parameters. If appropriate, totalvalues (as in the case of distance) are also formed for this andcompared with a prescribed upper limit. If it is detected during substepb) that one of the metric peripheral conditions is no longer fulfilledand the distance to the source node is not yet definitive for thedestination node, the iteration is aborted without success.

The Dijkstra routing algorithm determines in the example (see FIG. 8)the following edge sequence, drawn heavily in black, starting at thenode 21 and ending at the node 17:

That is to say the Dijkstra routing algorithm determines the sequence F1

{Link#: from-node#—to-node#}:

57:21-5, 8:5-1, 3:1-3, 15:3-9; 26:9-7; 23:7-8; 14:8-2; 11:2-6; 42:6-16;47:16-17;

The spokes and bypass exceptions are removed from this sequence F1, i.e.all those edges j for which the following applies:

LinkTable[j] .Type = “Spoke” or “Bypass”, also:

57:21-5, 15:3-9; 14:8-2; 42:6-16; 47:16-17;

From this {Link#: from-node#—to-node#}—sequence F1, a{Node#,Link#,Node#} sequence F2 is produced by formal re-ordering byexchanging from-node# and Link#, and the to-node# values are removed,with the exception of the very last one. In this way, the sequence F2,comprising n=5 edges and n+1=6 nodes, is obtained.

The method steps for forming the routing information, i.e. for acquiringa DTL stack, correspond to those of the first exemplary embodiment.These are repeated below, but adjusted to the designations used here andto the preliminary operations:

That sequence F3 whose nodes are never descending in terms of theirhierarchy levels are determined:

bhrl := FALSE; // bhrl is an abbreviation for: // Below Highest ReachedLevel current_node := node−1; // i.e.: = Source Node S = 21current_node_level := level of the current_node; // i.e.:= NodeTable[S].Hier- // archyLevel

for i:=1 step 1 to n−1 do

if level of the node-i+1 is lower (in other words numerically larger)than current_node_level then

 if bhrl = FALSE then

determine that ancestor node of node-i+1 whose level is equal to thecurrent_node_level. To do this, the values

NodeTable[k] .AncestorNode

are navigated through starting with k=node-i+1 until a node a is arrivedat with

NodeTable[a] .HierarchyLevel=current-node_level.

Link-i, which is a DownLink, is replaced by the associated Logical GroupNode horizontal link (with the same aggregation token). The reciprocaluplink j is equal to LinkTable [link-i] .MyUplink.

The sequence {LinkTable [j] .InducedLink} is navigated through until aj* is arrived at with LinkTable [j*] .InducedLink =0.

j* is the searched-for horizontal edge. Replace node i+1 by the ancestornode a which has been determined. bhr1:= TRUE;

else

remove link-i and node-i+1 from the sequence.

end

else

if bhr1= TRUE then

Replace link-i by that assigned (induced) Uplink or else

Logical Group Node horizontal link j* which starts from the level givenby the current_node_level and leads to the node-i+1.

That is to say the sequence (LinkTable [j] .InducedLink} is navigatedthrough, starting at j=link-i until a j* is arrived at withLinkTable[j*] .FromNode = i* such that NodeTable-[i*] .HierarchyLevel =current-node_level.

Node-i+1 is retained unchanged in the sequence. bhr1 := FALSE;

else

Retain link-i and node-i+1 unchanged in the sequence.

end

current_node_level : = Level of the node-i+1;

end

Next i;

In the example, the sequence F3 comprising n=4 edges and n+1=5 nodes isobtained:

Node21, Link57, Node3, Link15, Node8, Link14, Node6, Link21, Node12

A sequence F4 is formed from the sequence F3 as follows: for i:=1 step 1to n−1 do

if link-i = Uplink then

add after link-i that ancestor node of source node S which is of thesame level as node-i+1. In turn, add after it that horizontal link Hwhich is assigned to the link-i by virtue of the identical aggregationtoken. (The RelationTbl is run through by LinkTable [j] .Induced Linkstarting at j₁=link-i as far as H with LinkTable [j] .InducedLink=0 andH=j_(s) is found).

end

next i;

The sequence F4 is as follows:

Node21, Link57, Node12, Link20, Node3, Link15, Node8, Link14, Node6,Link21, Node12

A sequence of DTLs is formed from F4 in that the sequence F3 is brokenup after each uplink. The node numbers are replaced by the relevant nodeIDs and the edge numbers by the relevant port IDs. The uplink detectionis effected by virtue of LinkTable[j] Induced Link unequal to 0:.

1. DTL:

(NodeTable[21] NodeId, LinkTable[57] .PortId),

// Node in Atlanta of network A, Uplink U_(AB2)

2. DTL:

(NodeTable[12] .NodeId, LinkTable[20] .PortId,

// Network A, h_(AB2)

NodeTable[3] .NodeId, LinkTable[15] .PortId,

// Network B, h_(BC2)

NodeTable[8] .NodeId, LinkTable[14] .PortId,

// Network C, h_(BC1)

NodeTable[6] .NodeId, LinkTable[21] .PortId,

// Network B, h_(AB1)

NodeTable[12] .NodeId, 0) // Network A

The routing information which is obtained then has to be converted intoDTL information elements (IE) in compliance with the PNNI v1.0signalling protocol.

Refilling of DTL stack by the Entry Border switching node:

If a connection set-up message arrives at an entry border switching nodeS′ in a new peer group, the received DTL stack is filled again, i.e. aroute section is determined and is described by means of routinginformation (DTL). This route section corresponds to that hierarchicallyhigher link (this can be either a horizontal link or an uplink) which isreferenced by the current transit pointer of the uppermost DTL in thestack. It will be referred to below as Link-to-D′.

The starting point of the route section is S′ itself. The end point ofthe route section is the node D′, which is determined as follows.

The entry which follows the current transit pointer of the uppermost DTLin the stack contains the node ID of D′. However, if there is nosubsequent entry in it, this is found in the next lowest DTL in thestack, subsequent to the respective current transit pointer etc.

If such a subsequent entry is not contained even in the lowest DTL inthe stack, D′ is defined by means of a search term which is removed fromthe Called Party Number information element.

S′ searches through its topology database starting at the hierarchicallylowest node in order to find that destination node D′ which can bereached with reference to the search term. If the target address is aso-called ANYCAST address which generally adapts a plurality of piecesof equipment/servers to different nodes, an advertisement scope of areachability information PTSE is also checked during the determinationof D′.

j* will be assumed to be the edge number of the Link-to-D′, and i* willbe the node number of D′. Determination of J* and i*:

All the edges j will be searched through, and j* will be determined forthe one for which the following correspondence applies:

NodeTable [LinkTable [j] .FromNode].NodeId = Node ID, referenced by CTPAND LinkTable [J] .PortId = Port ID uppermost in the stack, referencedby CTP uppermost in the stack.

The node number i* of the destination node D′ is thus LinkTable [j*].ToNode.

Reduction of the topology graph:

As soon as the node S′ has determined the node D′, it excludes certainnodes and edges from the topology graph known to it (as describedabove). If a further rerouting attempt is made after the messageindicating nonfulfilment of the communications condition (crankback), italso removes all those nodes together with contact edges which hadpreviously proven to be blocking.

However, the following also applies:

The nodes together with contact edges of hierarchy level greaterthan/equal to that of the node D′ are removed, excepting D′ itself andexcepting all uplinks which end in D′ and which have the sameaggregation token as Link-to-D′. That is to say excepting the node i*and all the edges j for which the following applies:

LinkTable [j] .ToNode = i* AND LinkTable [j] .AggregationToken =LinkTable [j*].AggregationToken

Based on the topology graph which has been produced in this way, a bestroute from S′ to D′ (in other words from the node number whichrepresents the simple node S′ to i*) is determined, and DTL informationelements are derived from it, as described above. The DTL stack isfilled up again in the SET-UP message with the DTLs acquired in thisway, with the exception of the very last DTL to be acquired (whosecontent is already contained in the DTL received at the top of thestack).

3. Exemplary Embodiment (Grouping of Narrowband Networks)

A third exemplary embodiment (see FIGS. 9 to 14) relates to a consortiumof a plurality of narrowband network providers A, B, C which haveconnected their networks to form one communications network (see FIG.9). Each individual subnetwork is, viewed independently, a peer groupwhich is lowest in the hierarchy. Above it there is precisely onecommon, hierarchically higher peer group in which each subnetwork isrepresented as a logical node.

The significance of this concept is that the switching node of aspecific subnetwork only has to have precise knowledge of its ownnetwork topology. With regard to the other subnetworks, they only knowthe “rough” network topology of the communications network, in otherwords the way in which they are reciprocally interconnected.

Accessibility: each switching node of a specific subnetwork knows whichterminals (call numbers) are connected to each of the nodes of its ownsubnetwork. In addition, it knows which terminals (call numbers) areconnected to each external subnetwork—by means of summarized entries.

Three different network operators A, B and C connect their subnetworksreciprocally with physical lines (p):

in Dresden (DD): A to C, and B to C,

in Hamburg (HH): B to C,

in Munich (M): A to B,

in Cologne (K): A to B and B to C.

The dotted line in FIG. 9 designates the physical course of a routewhich is to be determined, starting in the source switching node(Munich) of the subnetwork A and ending in the destination switchingnode (Stuttgart), also of the network A.

In analogy to the PNNI concept, each of the subnetworks A, B, C can berepresented as a complex logical node, i.e. comprising an imaginarynucleus in the center of a large circle, ports at the edge of a largecircle, port-nucleus connecting lines (spokes), port-port connectinglines (bypass exceptions) (see FIG. 5).

In the source switching node or the device performing the formation ofthe routing information, the following graph G1, a so-called high-levelgraph (HL) is stored (see FIG. 6):

Edge HL-1 : Node HL-1 -Node HL-2

Edge HL-2 : Node HL-1 -Node HL-3

Edge HL-3 : Node HL-1 -Node HL-4

Edge HL-4 : Node HL-1 -Node HL-5

Edge HL-5 : Node HL-1 -Node HL-6

Edge HL-6 : Node HL-1 -Node HL-5

Edge HL-7 : Node HL-2 -Node HL-6

Edge HL-8 : Node HL-2 -Node HL-11

Edge HL-9 : Node HL-3 -Node HL-10

Edge HL-10 : Node HL-4 -Node HL-9

Edge HL-11 : Node HL-5 -Node HL-14

Edge HL-12 : Node HL-6 -Node HL-15

Edge HL-13 : Node HL-7 -Node HL-11

Edge HL-14 : Node HL-7 -Node HL-10

Edge HL-15 : Node HL-7 -Node HL-9

Edge HL-16 : Node HL-7 -Node HL-8

Edge HL-17 : Node HL-8 -Node HL-9

Edge HL-18 : Node HL-8 -Node HL-13

Edge HL-19 : Node HL-12 -Node HL-13

Edge HL-20 : Node HL-12 -Node HL-14

Edge HL-21 : Node HL-12 -Node HL-15

the connecting lines (links, edges) are designated from small nodenumber to relatively large node number.

This graph G1 is to be entered in the same way into the switching nodesof the subnetworks involved. Each nucleus node, each port node and eachedge thus receives a unique number. This number makes it possible foreach switching node to understand the same thing from the relevant nodenumber or edge number.

Each switching node receives a marker relating to nucleus or port, eachedge receives a marker in the form “spoke”, “bypass exception” or else“horizontal”. In addition, each switching node receives its geographicalcoordinates. Each nucleus node receives, in a suitable summarized form,specifications indicating which terminals are internal subscribers ofthis subnetwork. Each switching node and each edge receives a high-levelmarker HL.

Parallel to this, each switching node of a quite specific subnetworkreceives a topology graph G2 (low-level graph LL) of this specificsubnetwork, for example each switching node of the subnetwork A receivesthe following topology graph (see FIG. 12):

Edge LL-1 : Node LL-1 -Node LL-2

Edge LL-2 : Node LL-1 -Node LL-4

Edge LL-3 : Node LL-1 -Node LL-5

Edge LL-4 : Node LL-2 -Node LL-3

Edge LL-5 : Node LL-3 -Node LL-4

Edge LL-6 : Node LL-4 -Node LL-5

Edge LL-7 : Node LL-4 -Node LL-6

Edge LL-8 : Node LL-6 -Node LL-7

Edge LL-9 : Node LL-6 -Node LL-8

Edge LL-10 : Node LL-7 -Node LL-8

Edge LL-11 : Node LL-7 -Node LL-9

Edge LL-12 : Node LL-8 -Node LL-9

The edges are designated from small node number to relatively large nodenumber. Each switching node and each edge is marked with “low-level”=LL.

The source switching node combines the high-level graph and thelow-level graph by deleting its own high-level port nodes, its ownhigh-level nucleus node in the graph, and its own spoke and bypassexception edges. In contrast, the horizontal edges which lead away fromthese port nodes of said node are modified in such a way that they nowlead away from the switching nodes in accordance with the low-levelgraph. They continue to be marked as HL.

Each port of a specific subnetwork has topology information as to whichHL nodes represent its own subnetwork. Each port also knows, with regardto the HL port nodes of its own subnetwork, the respectively assigned LLnodes.

The graph G3 which is combined in this way is as follows:

Edge HL-1 : Node HL-1 -Node HL-2

Edge HL-2 : Node HL-1 -Node HL-3

Edge HL-3 : Node HL-1 -Node HL-4

Edge HL-4 : Node HL-1 -Node HL-5

Edge HL-5 : Node HL-1 -Node HL-6

Edge HL-6 : Node HL-1 -Node HL-5

Edge HL-7 : Node HL-2 -Node HL-6

Edge HL-8 : Node HL-2 -Node HL-11

Edge HL-9 : Node HL-3 -Node HL-10

Edge HL-10 : Node HL-4 -Node HL-9

Edge HL-11 : Node HL-5 -Node LL-6

Edge HL-12 : Node HL-6 -Node LL-1

Edge HL-13 : Node HL-7 -Node HL-11

Edge HL-14 : Node HL-7 -Node HL-10

Edge HL-15 : Node HL-7 -Node HL-9

Edge HL-16 : Node HL-7 -Node HL-8

Edge HL-17 : Node HL-8 -Node HL-9

Edge HL-18 : Node HL-8 -Node LL-9

Edge LL-1 : Node LL-1 -Node LL-2

Edge LL-2 : Node LL-1 -Node LL-4

Edge LL-3 : Node LL-1 -Node LL-5

Edge LL-4 : Node LL-2 -Node LL-3

Edge LL-5 : Node LL-3 -Node LL-4

Edge LL-6 : Node LL-4 -Node LL-5

Edge LL-7 : Node LL-4 -Node LL-6

Edge LL-8 : Node LL-6 -Node LL-7

Edge LL-9 : Node LL-6 -Node LL-8

Edge LL-10 : Node LL-7 -Node LL-8

Edge LL-11 : Node LL-7 -Node LL-9

Edge LL-12 : Node LL-8 -Node LL-9

Based on this topology graph G3, the respective source switching node ofthe subnetwork A activates a Dijkstra best route calculation andreceives a node-edge-node sequence, starting in the example at thesource switching node (Munich) and ending at the prescribed destinationswitching node (Stuttgart).

Munich=Kn.LL-9, Ka.HL-11, Kn.HL-5, Ka.HL-4, Kn.HL-1, Ka.HL-3, Kn.HL-3,Ka.HL-9, Kn.HL-10, Ka.HL-14, Kn-HL-7, Ka.HL-13, Kn.HL-11, Ka.HL-8,Kn.HL-2, Ka.HL-7, Kn.HL-6, Ka.HL-5, Kn.LL-1, Ka.LL-1, Kn.LL-2=Stuttgart.(see FIG. 13)

The node-edge-node sequence which is determined in also entered into aconnection set-up message by means of a single information element.Before this connection set-up message has been sent for the first time,this information element is evaluated and processed. In this process,the first node (=source switching node) is deleted. The first edge isevaluated in order to determine the correct trunk line to the nextswitching node. In response, this edge is also deleted in theinformation element. The “old”, i.e. already evaluated, information isthus cut away.

If the connection set-up message arrives at a switching node, theuppermost node of the information element being a HL node (HP portnode), the receiving switching node must calculate a route section (inwhich exclusively LL nodes occur) by means of the subnetwork which hasbeen reached at that time, receive the corresponding node-edge sequenceinto the routing information element and correspondingly carry on withthe connection set-up.

Case A

The uppermost node is a LL node number (that of the receiving switchingnode). If no edge number has been received, the destination switchingnode is reached. Otherwise, the connection set-up request is sent on viathe edge which has arrived at the top. However, the uppermost nodenumber and uppermost edge number are cutaway beforehand.

Case B

The uppermost node is a HL node number (specifically a HL port nodenumber). This can be transferred to the LL node number of the receivingswitching node. It is tested whether the subnetwork which has beenreached at that time is to be exited again—either immediately or afterpassing through its own subnetwork. The subnetwork is exited again if,on searching through the received routing information element (from topto bottom), either a LL node is arrived at or a HL (port) node of anexternal network is arrived at. In such a case, the subnetwork is exitedimmediately if either a subnetwork-external HL node or an LL node (whichbelongs to an external subnetwork) is present as the second node numberfrom the top.

Case B.1

The network cannot be exited again. The entire contents of the receivedrouting information element are removed. The LL destination node isdetermined on the basis of a terminal called by means of a received“Called Party Number” information element. The best route to it iscalculated and the node-edge-node sequence which has been determined isinserted into the routing information element—with the exception of thevery first node and the very first edge.

Case B.2.1

The subnetwork is to be exited again immediately. The connection set-uprequest is passed on via the edge found at the top, but the uppermostnode, and likewise this uppermost edge, is cut away beforehand.

Case B.2.2

The subnetwork is to be exited again after passing through. On searchingthrough from top to bottom, the last HL node which belongs to its ownnetwork (HL exit port node) is found in the routing information element.This is transferred to the relevant LL node (LL exit node), and a bestroute to this switching node is calculated. The uppermost entriesincluding those of the HL exit port node are deleted from the receivedrouting information element. A best route is calculated starting at theinstantaneous switching node and ending at the LL exit node, and therelevant node-edge-node sequence is inserted at the top into the routinginformation element (during which of course the starting node and thestarting edge are omitted), and the connection set-up message is passedon via the omitted starting edge.

In this method for forming routing information, a subnetwork (heresubnetwork A) is exited in the direction of the set-up and entered lateragain starting at another port.

If, in addition to the described routing information element, aninformation element (crankback; similar to that in PNNIv1.0) forinterrupting the display is inserted, said element being carried alongin a RELEASE message of the interruption in order to report a blockingnode, targeted rerouting can be initiated, during which it is guaranteedthat the blocking node is not searched for again. The blocking LL isreported. The RELEASE message is sent back as far as the sourceswitching node or else to the entry node in the subnetwork of theblocking node. Here, the Dijkstra algorithm is carried out again, basedon a topology graph in which the blocking node together with contactedges have been removed.

This is possibly repeated many times, i.e. at each further blockagethere is a report back to the node by RELEASE message, but then at thenext determination of a route the basis used is a topology graph inwhich all the previously reported blocking nodes together with contactedges are removed.

If all (a limited number) of these rerouting attempts fail, it ispossible to go back to the preceding network by RELEASE message, thecrankback information element containing a HL (nucleus) node. In thepreceding network, the RELEASE message should be sent back to the entrynode. Only there should the Dijkstra algorithm be carried out again, thereported blocking HL nucleus node, together with all the associated HLport nodes, being previously removed in the topology graph in order toensure that the blocking network is entirely avoided at the nextattempt.

During the determination of the route, the geographical coordinates ofthe (LL and HL) nodes, which are known, are resorted to. The route whichis found by means of the Dijkstra algorithm is a route with the shortestgeographical distances. The geographical distances can be represented ontariff zones, with the result that there is a cost reduction for theconnection set-up when the distance is short. When determining routes itis irrelevant whether the destination subscriber is connected to thecommunications network itself or is to be linked to an interface to anadjacent subnetwork outside the communications network.

The invention is not limited to the particular details of the methoddepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described method withoutdeparting from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A method for forming routing information for aconnection set-up, satisfying adjustable communications conditions, froma source switching node to a destination switching node, in acommunications network having at least two subnetworks with switchingnodes and connecting lines which connect the at least two subnetworks,subnetworks being connected to one another via connecting lines andtopology information on the subnetwork to which the source switchingnode belongs, and at least on an interconnection of the subnetworksbeing stored so as to be available for said source switching node,comprising the steps of: selecting a subset of the switching nodes andconnecting lines which satisfies the communications conditions and whosetopology information is stored in the source switching node; determininga route using elements of this set, which, in a direction from thesource switching node to the destination switching node, leaves at leastone subnetwork once and returns to said at least one subnetwork, andforming routing information from the route which has been determined. 2.The method as claimed in claim 1, wherein based on hierarchicallyorganized switching nodes which are represented by logical nodes ofdifferent orders, the route is determined with exclusion of parts of thehierarchy.
 3. The method as claimed in claim 1, wherein a subnetworkwhich contains the destination switching node is determined in thesource switching node from the topology information.
 4. The method asclaimed in claim 1, wherein a route and a description of a route isdetermined in accordance with principles of a Private Network NodeProtocol.
 5. The method as claimed in claim 4, wherein based on topologyinformation, at least one subnetwork is represented by a complexswitching node.
 6. The method as claimed in claim 5, wherein a complexswitching node is defined by at least one port node, a nucleus node,port-nucleus connecting lines and port-port connecting lines.
 7. Themethod as claimed in claim 6, wherein connecting lines relating toport-nucleus links and port-port links are removed during formation ofthe routing information.
 8. The method as claimed in claim 1, whereinconnecting lines between two switching nodes in both traffic directionsare handled independently.
 9. The method as claimed in claim 1, whereinthe route is determined according to a Dijkstra algorithm.
 10. Themethod as claimed in claim 1, wherein when the communications conditionsfor a route which has already been determined are not fulfilled, atleast one of a respective switching node and respective connecting lineare signalled to the source switching node.